We propose that there exists mathematical knowledge for teaching (MKT) specific to visual representations (VRs), abbreviated MKT-VR. We define a VR as a graphic creation, such as a diagram or drawing, which illustrates quantities and shows quantitative relationships or which illustrates geometric properties of figures and shows geometric relationships. A teacher with strong MKT-VR will, for example, be able to use and understand VRs in his/her own problem solving and will have mathematical knowledge specific to teaching students to use, analyze, and solve problems with VRs. The Visual Access to Mathematics (VAM) project seeks to help teachers understand the value of VRs, specifically when teaching and learning ratio and proportional reasoning content. This chapter lays out a theoretical framework that we anticipate using to guide and benchmark future research. [For the complete volume, "Pedagogical Content Knowledge in STEM: Research to Practice. Advances in STEM Education," see ED613708.]